The present invention relates to analytical systems and, more particularly, to gas chromatography systems. A major objective of the invention is to provide a compact, thermally agile, gas chromatography system.
Gas chromatography (GC) is a method of separating volatile organic and inorganic sample components. In GC, a sample is progressively heated through the boiling points of its components so that they can be differentially swept through a sorbent-coated column by a carrier gas. Components are separated according to the extent they preferentially bind to the sorbent material. To ensure maximum resolution, spatial temperature gradients in the column should be minimized. The required isothermal conditions are achieved by careful design of the heating system and oven geometry and by use of a fan to promote thorough mixing of the air that circulates past the column.
To ensure repeatability and comparability of results with standard retention-time tables, a GC oven must regulate temperature to match a selected demand ramp (or, more precisely, "demand function"). A demand ramp is a prescribed temperature-versus-time function that generally includes one or more periods of constant positive slope. In addition, a demand ramp can include one or more constant temperature periods to stabilize conditions at the beginning and/or end of a run, or to dwell temperature that favors separation of closely eluting components, thus, creating, so to speak, a chromatographic "sweet spot.
Typically, oven temperature is monitored during a ramp so that it can be compared with the temperature assigned at any given instant by the demand ramp. If the measured temperature is below the demand temperature, the power to the heater is increased. If the measured temperature is above the demand temperature, the power to the heater is decreased.
If the temperature remains high after power to the heater has been decreased to zero, the error cannot be corrected by further control of the heater. This problem typically arises when heat contributed by uncontrolled heat sources, even with the heater power off, exceeds that required by the demand ramp.
There are several such uncontrolled heat sources. Capillary inlets and outlets are continuously heated to avoid condensation; thus, these inlets and outlets function as uncontrolled heat sources that can raise the oven temperature even while the main heat source is off. In addition, heat dissipated by a stirring fan is an uncontrolled heat source. Even the heating element, to the extent that its thermal mass prevents instantaneous control of its output, can be considered to be in part a source of uncontrolled heat.
Furthermore, heat remaining in oven insulation from prior sample runs functions as an uncontrolled passive heat source. This last uncontrolled heat source can be addressed by allowing a longer cool down period between runs. Note that insulator heat is lower for a first run than for subsequent runs. This causes the first run to not be precisely comparable to subsequent runs. This phenomenon is known as the "first run effect". Often the first run is simply discarded at the expense of instrument productivity.
Long cooldown periods are undesirable because they lengthen the sample cycle time, further reducing instrument productivity. Most GC ovens employ ventilation in some form to increase the rate of heat removal during cool down. Furthermore, the availability of ventilation during a ramp means that heat remaining in the insulation does not have to be fully removed between runs. Thus, ventilation reduces cooldown time in two ways: 1) the availability of ventilation for ramp temperature control reduces the amount of cooling required between runs; and 2) the use of ventilation during cooldown decreases the time required to achieve a required amount of cooling.
A ventilation system used for ramp control must be carefully designed so that the ventilation does not introduce temperature gradients in the column. To minimize local temperature deviations at the column, the ventilation can be mixed with circulating air in a separate stirring chamber at the rear of the oven enclosure.
In one exemplary oven, intake and exhaust vents on the rear face of the oven enclosure can be used to cool the air in a stirring chamber to the rear of a main "column" chamber. Air from the stirring chamber is then circulated with air in the column chamber. The main chamber and stirring chamber are separated by a partition. The partition is spaced from the top and side faces of the oven enclosure to define an annular aperture through which stirred air flows to the main chamber. An aperture through the center of the partition provides a return path to the stirring chamber. A fan in the stirring chamber mixes ventilation flow with circulation flow and forces the mixed air out through the annular aperture.
Even with the use of ventilation during cooldown to increase the rate of cooling and the use of ventilation for near-ambient temperature control to reduce the amount of cooling required, instrument performance and productivity can be limited. A typical ramp from near-ambient temperature to a maximum of about 400.degree. C. consumes about one-half hour, while another half hour can be required for cooldown for a full-hour cycle time. Near ambient temperature control is not generally available within 10.degree. C. of ambient. Given the insatiable demand for GC performance and productivity, control at lower temperatures as well as faster ramp and cooldown times are sought.
To provide a fast run, a demand ramp can have a steep positive slope to a maximum temperature, at which the slope drops suddenly to zero (followed by a negative slope during cooldown). The fast ramp requires a heating element that is much hotter (e.g., 100.degree. C. hotter) than the temperature at the column. When the heating element is turned off at the maximum demand column temperature (e.g., 400.degree. C.), the heating element continues to glow for several seconds due to its thermal mass. The resulting excess heat causes the column temperature to surpass the demand maximum by several degrees. A similar overshoot can occur at an intermediate temperature; a user can select a ramp that has a sudden reduction of slope at an intermediate temperature selected to promote separation of otherwise difficult-to-separate sample components.
One problem with thermal overshoot is that the oven temperature temporarily deviates from the demand ramp; this makes it difficult to compare a chromatogram obtained using one manufacturer's GC system with those from other manufacturers and with standard retention-time tables. Another problem that is not widely recognized in the art can be an even greater concern. During overshoot, the oven temperature is not controlled and is therefore variable from run to run according to such factors as external temperature and first run effect. Thus, thermal overshoot impairs comparison of chromatograms even across successive runs from the same instrument.
One approach to minimizing thermal overshoot would be to "smooth" or "round" demand ramp corners (slope transition points). Since there are many ways of rounding a corner, this approach introduces another variable along which instruments can differ; this makes inter-instrument comparisons and comparisons with standardized retention-time tables problematic.
In addition to problems with instrument productivity and with reproducibility due to temperature control, it is well recognized that GC ovens are undesirably large. GC ovens typically constitute over half the volume of a GC system, which in turn consumes valuable laboratory bench space. What is needed is a compact, thermally agile GC system that provides for fast ramps and quick cooldowns, closer-to-ambient temperature control, and minimal thermal overshoot.